Electromagnetic radiation absorbing materials are used in a variety of applications. They are commonly used in Electromagnetic Compatibility/Electromagnetic Interference (EMC/EMI) test cells to eliminate reflection and interference during testing. Electromagnetic radiation absorbers are also used in electromagnetic anechoic chambers for testing high frequency radar, in antennas and in Low Observable (LO) structures. The increase in consumer electronics that broadcast, such as cellular telephones and portable computers, have created a new need: the suppression of stray electromagnetic signals in airplanes and near airports to prevent interference with airport radar, communications and automated landing systems. Intelligent Vehicle Highway Systems (IHVS) may also require the suppression of stray electromagnetic signals to prevent multi-path and other types of interference.
Previously, electromagnetic absorbers used only either the electric or the magnetic properties of a material to attenuate the electromagnetic fields. Electric absorption is normally achieved by introducing lossy material, often carbon, to a low dielectric constant material. Examples of this approach include carbon loaded foam and carbon loaded honeycomb. An alternate method is to use specific patterns of the lossy material to achieve a Debye relaxation of the dielectric constant. See U.S. patent application Ser. No. 07/890,757 entitled METHOD FOR MAKING A MATERIAL WITH ARTIFICIAL DIELECTRIC CONSTANT, now U.S. Pat. No. 5,385,623, the disclosure of which is incorporated by reference. The relaxation of the dielectric constant produces a loss in the material that can be accurately controlled in both magnitude and frequency.
Magnetic loss is generally achieved by using a material that exhibits a natural magnetic loss mechanism. Ferrites are a class of material that exhibit this type of loss and are often used in absorbing materials. However, in the frequency range where the ferrite's loss is useful, the real part of their relative permittivity and real part of their relative permeability are very different from each other. The result is that the material's impedance is not close to the impedance of free space and a significant part of the incident energy reflects off the surface. Only when the interference between the surface reflection and reflection from the surface underneath the ferrite cancel each other does the material exhibit its full loss. Therefore, absorbers which use ferrites are only effective over a very limited band of frequencies.
The performance of electromagnetic absorbing materials can be improved through grading the electric and magnetic properties within the material and/or by shaping the material. However, even with these techniques, the current state of the art of electromagnetic absorbers results in materials that are either very thick, or work only over a narrow band of frequencies. For example, carbon-loaded, foam pyramids used in EMC/EMI test cells are approximately 10 feet long and require ferrite tiles on their base to achieve 10 dB of absorption from 10 MHz to 1 GHz. The size and weight of the pyramids places special requirements on room size and the load bearing capacity of the walls and ceiling.
Moreover, absorbing an electromagnetic wave incident from free space onto the material involves two important steps:
1. Getting the majority of the power of wave to enter the material; and PA1 2. Dissipating the power of the wave as heat using the loss mechanisms in the material.
The first condition is controlled by the thickness of the material, the frequency of the incident wave and the intrinsic impedance of the material. The intrinsic impedance of the material is given by: ##EQU1## where .mu. is the permeability of the material, .mu..sub.0 is the permeability of free space, .mu..sub.r =.mu./.mu..sub.0 is the relative permeability, .di-elect cons. is the permittivity of the material, .di-elect cons..sub.0 is the permittivity of free space, .di-elect cons..sub.r =.di-elect cons./.di-elect cons..sub.0 is the relative permittivity of the material and .eta.=(.mu..sub.0 /.di-elect cons..sub.0).sup.1/2 =377.OMEGA. is the impedance of free space. Note that the permittivity and permeability of materials are generally complex and frequency dependent, i.e.: EQU .mu.=.mu.'(f)-j.mu."(f) (2) EQU .di-elect cons.=.di-elect cons.'(f)-j.di-elect cons."(f) (3)
where j=(-1).sup.1/2 is the imaginary unit. When the impedance of the material is the same as free space, all of the power in the incident wave enters the material regardless of the thickness or operating frequency. Clearly, the impedance of the material will be equal to free space when .mu..sub.r =.di-elect cons..sub.r.
The second condition is controlled by the loss that the electromagnetic wave experiences once it has entered the material. The power dissipated, P.sub.L, is roughly proportional to an exponential function: EQU P.sub.L .varies.e.sup.-2.gamma.l (4)
where l is the thickness of the material and .gamma. is the complex propagation constant given by: EQU .gamma.=j2.pi.f.sqroot..mu..di-elect cons. (5)
and f is the operating frequency of the wave. The loss, which is the real part of .gamma., comes from the imaginary parts of the permittivity and/or permeability, .mu." and .di-elect cons.". So, to attenuate the wave, the material should have large imaginary parts of the permittivity and/or permeability.
Thus the ideal absorbing material is one which has an impedance equal to free space and is as lossy as possible. This give the conditions: EQU .mu..sub.r (f)=.epsilon..sub.r (f) (6)
and EQU .mu..sub.r (f).apprxeq..di-elect cons..sub.r (f).fwdarw..infin.(7)
For this ideal material, increasing the imaginary parts of the permittivity and permeability decreases the thickness of the absorbing material required to achieve a desired level of performance. For practical absorber design, the above criteria are required over a broad but finite band of frequencies.
Hexcel has produced materials with controlled, frequency dependent anisotropic dielectric properties using Debye relaxations (U.S. Pat. No. 5,385,623). Magnetic loss which exhibits Debye-like behavior can be obtained in one of two ways: (1) using natural, lossy magnetic materials, such as ferrites; or (2) using the skin-effect of permeable, conducting materials (L. D. Landau and E. M. Lifshitz, Electrodynamics of Continuous Media, Pergamon Press, Oxford, 1984) as in a laminated magnetic transformer core. The Debye relaxation of the real part of either the permittivity or permeability produces an imaginary part that contributes to power dissipation in the material.
Since both electric and magnetic materials can exhibit controlled anisotropic Debye-like behavior, it is possible to design a material that has an impedance that is anisotropic and essentially matched to free space over the desired band of frequency, and has both magnetic and electric loss mechanisms. It would be desirable to provide a composite material that combines synthetic dielectric materials with a lossy magnetic material (either natural or skin-effect) to reduce the scattering from composite structures. The amount of materials, shaping and material properties are selected by the designer so that the relative permittivity and permeability are substantially the same so that the impedance at the given angle matches that of free space at that angle over the frequency range of interest, with the composite material exhibiting both electric and magnetic loss mechanisms. The performance of the composite material can be enhanced by grading the properties of the material and/or shaping the material, just as with prior art materials.
The particular structure scattering problem addressed by this invention is the scattering of electromagnetic waves at shallow angles from dielectric surfaces.
The shallow angle problem arises, for instance, in the design of missile radomes for gimballed antennas. Because of stringent aerodynamic requirements, these radomes must have very high aspect ratios. Therefore, when the radar is looking forward, its signal traverses the radome wall at a very shallow angle. The TM component of the wave (electric field is parallel to the plane of incidence) transmits readily through the dielectric, owing to the onset of the Brewster angle phenomenon, while the TE component (electric field perpendicular to the plane of incidence) reflects very strongly off the dielectric interface. The net result is that the radar signal suffers severe depolarization and a significant internal echo in the TE polarization, leading to the so called "flashlobes".
A similar problem arises in the construction of composite panel absorbers for low observable applications. Such panels always contain an outer dielectric skin of finite thickness dictated by mechanical and environmental requirements. At low frequencies, the skin is electrically thin and has little effect on the design. However, at high frequencies, as the skin thickness approaches the quarter wave anti-resonance, the skin becomes a strong scatterer and in fact becomes the limiting parameter in the design. Just as in the case of the radome, this problem gets exacerbated when the angle of incidence moves off the normal. An absorber panel with good performance at normal incidence remains very good for the TM polarization as the angle of incidence moves off the normal (due to the Brewster angle) but it degrades very rapidly for the TE polarization (since then less and less energy penetrates the skin to get absorbed in the panel).
Finally, the same problem arises in the area of automotive radar. Multi-path scattering off the road at shallow angles, which causes undesirable and dangerous interference, is always worse for the TE polarization.
These applications highlight the need for a material whose polarization response at shallow angles can be tailored to the particular need. To do this requires synthetic magnetodielectrics.
The Brewster angle phenomenon for TM waves arises from an impedance matching effect at the air-dielectric interface as the angle of incidence moves away from the normal. An identical phenomenon would occur for the TE polarization if instead of an air-dielectric interface there was an air-magnetic interface. It can be readily calculated that near 450.degree. incidence, a material with a permeability twice as high as its permittivity can become perfectly transparent. In fact, any amount of magnetic permeability added to a dielectric would be an improvement and serve to reduce the TE reflection coefficient. Of course, any action taken to improve the TE polarization will tend to worsen the TM response. However, since the TM polarization is so good to begin with, there is ample room to effect this trade-off.
In view of the foregoing, it is desirable to provide a method for forming composite electromagnetic absorbers that provide increased electromagnetic radiation absorption and which are thinner and/or lighter than those of the prior art. More particularly, it is desirable to provide synthetic dielectric materials which are combined with either synthetic magnetic materials or magnetically lossy materials in such a way that the impedance of the composite material is substantially matched over the desired range of angles and frequency. The match in impedance allows the majority of the electromagnetic fields to enter the material so that the electric and magnetic loss components are able to absorb the electromagnetic energy.